The following case is in some respects more interesting. A pied variety
of the raven, with the head, breast, abdomen, and parts of the wings
and tail-feathers white, is confined to the Feroe Islands. It is not
very rare there, for Graba saw during his visit from eight to ten
living specimens. Although the characters of this variety are not quite
constant, yet it has been named by several distinguished ornithologists
as a distinct species. The fact of the pied birds being pursued and
persecuted with much clamour by the other ravens of the island was the
chief cause which led Brunnich to conclude that they were specifically
distinct; but this is now known to be an error. (40. Graba, Tagebuch
Reise nach Faro, 1830, ss. 51-54. Macgillivray, History of British
Birds, vol. iii. p. 745, Ibis, vol. v. 1863, p. 469.) This case
seems analogous to that lately given of albino birds not pairing from
being rejected by their comrades.
In various parts of the northern seas a remarkable variety of the
common Guillemot (Uria troile) is found; and in Feroe, one out of every
five birds, according to Graba s estimation, presents this variation.
It is characterised (41. Graba, ibid. s. 54. Macgillivray, ibid. vol.
v. p. 327.) by a pure white ring round the eye, with a curved narrow
white line, an inch and a half in length, extending back from the ring.
This conspicuous character has caused the bird to be ranked by several
ornithologists as a distinct species under the name of U. lacrymans,
but it is now known to be merely a variety. It often pairs with the
common kind, yet intermediate gradations have never been seen; nor is
this surprising, for variations which appear suddenly, are often, as I
have elsewhere shewn (42. Variation of Animals and Plants under
Domestication, vol. ii. p. 92.), transmitted either unaltered or not
at all. We thus see that two distinct forms of the same species may
co-exist in the same district, and we cannot doubt that if the one had
possessed any advantage over the other, it would soon have been
multiplied to the exclusion of the latter. If, for instance, the male
pied ravens, instead of being persecuted by their comrades, had been
highly attractive (like the above pied peacock) to the black female
ravens their numbers would have rapidly increased. And this would have
been a case of sexual selection.
With respect to the slight individual differences which are common, in
a greater or less degree, to all the members of the same species, we
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